Ischemia detection

ABSTRACT

Techniques for detection and treatment of myocardial ischemia are described that monitor both the electrical and dynamic mechanical activity of the heart to detect and verify the occurrence of myocardial ischemia in a more reliable manner. The occurrence of myocardial ischemia can be detected by monitoring changes in an electrical signal such as an ECG or EGM, and changes in dynamic mechanical activity of the heart. Dynamic mechanical activity can be represented, for example, by a heart acceleration signal or pressure signal. The electrical signal can be obtained from a set of implanted or external electrodes. The heart acceleration signal can be obtained from an accelerometer or pressure sensor deployed within or near the heart. The techniques correlate contractility changes detected by an accelerometer or pressure sensor with changes in the ST electrogram segment detected by the electrodes to increase the reliability of ischemia detection.

FIELD

[0001] The invention relates to cardiac health and, more particularly,to techniques for detection of myocardial ischemia.

BACKGROUND

[0002] Myocardial ischemia, a leading cause of mortality, involvesoxygen starvation of the myocardium. Myocardial ischemia can lead tomyocardial infarction if left untreated. Early detection of myocardialischemia provides the opportunity for a wide range of effectivetherapies such as surgical revascularization, neural stimulation, anddrug delivery to reduce cardiac workload or improve cardiac circulation.Unfortunately, many episodes of myocardial ischemia do not causeexcessive pain or other noticeable warning signs, and often goundetected.

[0003] An electrocardiogram (ECG) or electrogram (EGM) presents a PQRSTwaveform sequence that characterizes the cyclical cardiac activity of apatient. The T-wave can be used to identify an ischemic condition. U.S.Pat. No. 6,016,443 to Ekwall et al., for example, describes animplantable ischemia detector that employs a repolarization sensor and apatient workload sensor to identify ischemic episodes. Therepolarization sensor detects T-wave amplitude or duration to identifyincreased heart rate. The workload sensor detects patient activity suchas exercise by monitoring body movement, muscle sounds, fluid pressurewaves, or metabolic changes. When the T-wave indicates an increasedheart rate, without a corresponding increase in workload, the detectoridentifies an ischemic condition.

[0004] The ST segment, also associated with the repolarization of theventricles, is typically close in amplitude to the baseline, i.e.,isoelectric amplitude, of the signal sensed between consecutive PQRSTsequences. During episodes of myocardial ischemia, the ST segmentamplitude deviates from the baseline. Accordingly, deviation in the STsegment is often used to identify an occurrence of myocardial ischemia.

[0005] U.S. Pat. No. 6,021,350 to Mathson, for example, describes animplantable heart stimulator having an ischemia detector that indicatesan ischemic condition based on elevation of the ST-segment above abaseline. Alternatively, the ischemia detector may rely on a measure ofheart activity or patient workload. The stimulator controls the rate ofstimulation based on the detection of ischemia using either of thealternative detection modes.

[0006] Unfortunately, the use of the ST segment as an indicator ofischemia can be unreliable. The ST segment may deviate from the baselinedue to other factors, causing false indications of myocardial ischemia.For example, the ST segment may deviate from the baseline due to changesin the overall PQRST complex, possibly caused by axis shifts, electricalnoise, cardiac pacing stimuli, drugs and high sinus or tachycardia ratesthat distort the PQRST complex. Consequently, the reliability of the STsegment as an indicator of myocardial ischemia can be uncertain.

[0007] U.S. Pat. No. 6,128,526 to Stadler et al. describes an ischemiadetector that observes variation in the ST segment to identify anischemic condition. To improve reliability, the detector is designed tofilter out ST segment variations caused by factors other than ischemia,such as axis shift, electrical noise, cardiac pacing, and distortion inthe overall PQRST complex.

[0008] Efforts to verify the reliability of the ST segment havegenerally proven complicated. Accordingly, there continues to be a needfor a simplified system capable of automatically and reliably detectingmyocardial ischemia.

SUMMARY

[0009] The invention is directed to techniques for more reliabledetection and treatment of myocardial ischemia. In particular, theinvention correlates electrical activity and dynamic mechanical activityof a heart to detect and verify the occurrence of myocardial ischemia ina more reliable manner.

[0010] The electrical activity may be represented by the ST segment. Thedynamic mechanical activity may be represented by a heart accelerationor pressure signal. Heart acceleration or pressure provides anindication of heart contractility. The term “contractility” generallyrefers to the ability of the heart to contract, and may indicate adegree of contraction. Heart contractility typically decreases duringischemic episodes.

[0011] Accordingly, the invention determines whether a change in the STsegment is accompanied by a corresponding change in the contractility ofthe heart. Correlation of changes in the contractility of the heart withchanges in the ST segment provides a more reliable indication ofischemia, reducing the incidence of false indications due to ST segmentchanges that are unrelated to ischemic conditions.

[0012] Changes in the ST segment can be detected from an ECG, EGM, orsubcutaneous electrode array (SEA). Changes in the dynamic mechanicalactivity of the heart can be obtained from an accelerometer or pressuretransducer. The accelerometer produces an acceleration signal indicativeof heart wall acceleration within a chamber of the heart. The pressuretransducer produces a pressure signal indicative of right ventricular,left ventricular, or arterial pressure, depending upon the location ofthe pressure transducer.

[0013] For the ST segment, the electrical signal can be obtained from aset of implanted or external electrodes. For dynamic heart activity, anaccelerometric signal can be obtained from an accelerometer deployedwithin or near the heart. The accelerometer transduces heartcontractions into one or more accelerometric signals. The pressuresignal can be obtained from a pressure transducer deployed within theheart or vasculature. Alternately, the pressure sensor could bepositioned around a blood vessel.

[0014] The accelerometer can be disposed at the distal tip of animplanted lead that is deployed within a chamber of the heart. Thepressure transducer can be realized by a cardiac pressure lead. A signalprocessing circuit can be used to detect drops in contractility duringmyocardial ischemia by comparing the accelerometric or pressure signalto a criterion such as a predetermined threshold.

[0015] The invention correlates contractility changes derived fromsignals generated by a lead tip accelerometer or cardiac pressure leadwith changes in the ST segment to increase the specificity of ischemiadetection. In particular, the utilization of a lead tip accelerometer orpressure lead in conjunction with electrical detection permitsdifferentiation between ST segment changes accompanied by changes incardiac contractility and ST segment changes without significant changesin cardiac contractility. Changes in cardiac contractility derived fromthe accelerometer or pressure lead provide another indication ofischemic conditions, and confirm the indication provided by the STsegment.

[0016] In one embodiment, the invention provides a method for detectingmyocardial ischemia, the method comprising obtaining a first signalindicative of dynamic mechanical activity of a heart, obtaining a secondsignal indicative of electrical activity of the heart, and detectingmyocardial ischemia based on both the first signal and the secondsignal. The invention also may provide computer-readable media carryinginstructions for performing the method.

[0017] In another embodiment, the invention provides a system fordetecting myocardial ischemia, the system comprising a first sensor thatgenerates a first signal indicative of dynamic mechanical activity of aheart, a second sensor that obtains a second signal indicative ofelectrical activity of the heart, and a processor that detectsmyocardial ischemia based on both the first signal and the secondsignal.

[0018] In an added embodiment, the invention provides a method fordetecting myocardial ischemia, the method comprising obtaining a firstsignal indicative of contractile activity of a heart, obtaining a secondsignal indicative of electrical activity of the heart, and detectingmyocardial ischemia based on both the first signal and the secondsignal. The invention also may provide computer-readable media carryinginstructions for performing the method.

[0019] In a further embodiment, the invention provides a system fordetecting myocardial ischemia, the system comprising means forgenerating a first signal indicative of contractile activity of a heart,means for obtaining a second signal indicative of electrical activity ofthe heart, and means for detecting myocardial ischemia based on both thefirst signal and the second signal.

[0020] The invention is capable of providing a number of advantages. Forexample, correlation of changes in heart contractility with changes inthe ST segment provide a more reliable indication of an ischemic event.In this manner, the invention is useful in increasing the specificity ofischemia detection, generally avoiding false indication of ischemicevents due to axis shifts, electrical noise, cardiac pacing stimuli,high sinus or tachycardia rates, or other factors that undermine theeffectiveness of a purely electrical detection technique. Also, theinvention is capable of improving sensitivity to ischemic episodes byallowing the detection of ischemia when either the mechanical or theelectrical signals are indicative of ischemia.

[0021] In addition, the invention can be useful in quantifying a degreeof ischemic tissue according to a degree of cardiac contractility and adegree of change in the ST segment. Moreover, the combination ofelectrical and mechanical monitoring of heart activity can aid indetermining the location of ischemic tissue. In particular, both theelectrical and mechanical signals can be monitored along multiple axes.The electrical signal may include multiple electrical signals obtainedfrom different lead sets, whereas an accelerometer may be sensitivealong two and perhaps three axes. Likewise, multiple accelerometers orpressure sensors can be used to achieve sensitivity along multiple axes.

[0022] The above summary of the invention is not intended to describeevery embodiment of the invention. The details of one or moreembodiments of the invention are set forth in the accompanying drawingsand the description below. Other features, objects, and advantages ofthe invention will be apparent from the description and drawings, andfrom the claims.

BRIEF DESCRIPTION OF DRAWINGS

[0023]FIG. 1 is a diagram illustrating an exemplary implantable medicaldevice in association with a heart.

[0024]FIG. 2 is a diagram illustrating another exemplary implantablemedical device in association with a heart.

[0025]FIG. 3 is a side view illustrating an implantable lead suitablefor incorporation of a lead-tip accelerometer.

[0026]FIG. 4 is a cross-sectional side view of the lead shown in FIG. 3.

[0027]FIG. 5 is a block diagram illustrating a system for detection ofischemia.

[0028]FIGS. 6A, 6B, and 6C are graphs illustrating the relationshipbetween electrical activity and heart acceleration within a canineheart.

[0029]FIGS. 7A, 7B, and 7C are graphs illustrating the relationshipbetween electrical activity and heart acceleration within a canine heartduring an episode of ischemia.

[0030]FIG. 8 is another graph illustrating changes in heart accelerationin the presence of ischemia.

[0031]FIG. 9 is a flow diagram illustrating a process for ischemiadetection.

[0032]FIG. 10 is a flow diagram illustrating another process forischemia detection.

[0033]FIG. 11 is a flow diagram illustrating a process for ischemiadetection in greater detail.

DETAILED DESCRIPTION

[0034]FIG. 1 is a diagram illustrating an implantable medical device(IMD) 10 in association with a heart 34. IMD 10 may be configured forboth monitoring and therapy of heart 34. For example, IMD 10 may includea pulse generator to deliver electrical stimulation to heart 34 for usein cardioversion or defibrillation. In accordance with the invention,IMD 10 obtains a signal indicative of dynamic mechanical activity ofheart 34, and an electrical signal indicative of electrical activity ofthe heart.

[0035] Using both signals, i.e., the electrical signal and the signalindicative of dynamic mechanical activity, IMD 10 detects the existenceof myocardial ischemia within heart 34. When both signals revealischemic conditions, IMD 10 indicates an ischemic episode. The signalindicative of dynamic mechanical activity corroborates the electricalsignal.

[0036] If ischemia is detected, IMD 10 can be configured to deliverappropriate therapy to alleviate its effects. The therapy may includedrug delivery, electrical stimulation, or both. In addition, accordingto some embodiments, IMD 10 may determine the location of ischemictissue and the severity of the ischemic condition, providing morespecific information that may be useful in selection of treatment.

[0037] IMD 10 may be generally flat and thin to permit subcutaneousimplantation within a human body, e.g., within upper thoracic regions orthe lower abdominal region. IMD 10 may include a hermetically sealedhousing 14 having a connector block assembly 12 that receives theproximal ends of one or more cardiac leads for connection to circuitryenclosed within housing 14. In the example of FIG. 1, connector blockassembly 12 receives a ventricular endocardial lead 28.

[0038] In some embodiments of the invention, ventricular endocardiallead 28, or other leads, may include an accelerometer to obtain a heartacceleration signal or a pressure transducer to obtain a pressuresignal. In other embodiments, a pressure signal can be obtained fromoutside a blood vessel, e.g., with the use of implantable blood vesselcuffs as described in U.S. Pat. Nos. 6,010,477 and 6,077,277 to Mieselet al.

[0039] Also, multiple accelerometers or pressure sensors can be used toachieve sensitivity along multiple axes. For instance, if a coronaryartery providing oxygen to the left side of the heart is occluded, theremay be a decrease in accelerometer- or pressure sensor-indicatedcontractility from a left-sided lead, but not necessarily from a lead inthe right ventricle. This may be particularly the case for anaccelerometer lead placed on the right ventricular free wall, which isnot mechanically coupled to the left ventricle, as well as a lead placedon the ventricular septum. Accordingly, multiple sensors may bedesirable for enhanced sensitivity.

[0040] An accelerometer will be generally described herein for purposesof illustration. Ventricular endocardial lead 28 may be, for example, abipolar, two wire lead equipped to sense electrical signals. Anaccelerometer can be incorporated adjacent a distal tip 30 of lead 28,and thereby deployed within heart 34. As will be described, housing 14may enclose circuitry for use in analyzing the heart acceleration signalproduced by the accelerometer, and electrical signals such as ECGs orEGMs obtained by IMD 10 to detect ischemia within heart 34.

[0041] To facilitate detection of electrical activity within heart 34,IMD 10 may include a plurality of EGM sense electrodes 16,18, 20, 22,24, 26. EGM sense electrodes 16, 18, 20, 22, 24, 26 may be arrangedsubstantially as described in U.S. Pat. No. 6,128,526, to Stadler etal., entitled “METHOD FOR ISCHEMIA DETECTION AND APPARATUS USING SAME,”the entire content of which is incorporated herein by reference. Forexample, electrodes 16, 18, 20, 22, 24, 26 may form a plurality of senseelectrode pairs that are integrated with the exterior of housing 12 ofIMD 10.

[0042] The sense electrode pairs can be used to obtain electricalsignals along one or more sensing axes to formulate one or more EGMsignals. The EGM signal obtained via sense electrodes 16, 18, 20, 22,24, 26, together with the heart acceleration signal provided by anaccelerometer or pressure transducer, can be used to detect ischemia, aswell as the degree of ischemia and the location of ischemic tissuewithin heart 34. The accelerometer provides an indication of the dynamicmechanical activity of the heart, which either reinforces or negates anindication of ischemia derived from a change in the electrical signal.

[0043] As an advantage, in addition to identification of ischemia, theheart acceleration signal can be used to measure other events indifferent frequency ranges. For example, the heart acceleration signalmay be monitored from 0 to 0.5 Hz for the patient's posture ororientation, from 1 to 5 Hz for the patient's activity, e.g., exercise,and from 5 to 100 Hz for the patient's heart acceleration. The frequencyrange for analysis of heart acceleration is the range useful inidentification of ischemia. Thus, the accelerometer may serve multiplepurposes. For example, by analyzing the pertinent frequency bands, theaccelerometer may be used to detect patient activity, patentorientation, and heart acceleration.

[0044] As further shown in FIG. 1, a programmer/output device 44 with anantenna 46 can be provided for wireless communication with IMD 10. IMD10 may include a telemetry circuit that transmits radio frequencymessages, which may include indications of ischemia and otherinformation to device 44. IMD 10 also may receive programminginformation via the telemetry circuit for modification of operationalparameters within the IMD.

[0045] Device 44 also may include a display for graphic or textualpresentation of information transmitted by IMD 10, as well as a visibleor audible annunciator that provides an indication of the detection ofischemia within heart 34. IMD 10 also may be equipped with an alarm fornotification of the patient in the event ischemia is detected. Also,device 44 may include a user input device, such as a keypad, by which aphysician may modify operational parameters for use in programming IMD10 for diagnosis or treatment.

[0046]FIG. 2 is a diagram illustrating another IMD 48 in associationwith a human heart 34. In particular, IMD 48 may be configured toprovide electrical stimuli to heart 34 for defibrillation. IMD 48 maygenerally conform to the defibrillation system described in theabove-referenced U.S. Pat. No. 6,128,526. In the example of FIG. 2, IMD48 includes an outer housing 54 that functions as an electrode, alongwith a set of electrodes 52, 56, 58 provided at various locations on thehousing or connector block 50.

[0047] IMD 48 may include leads 60, 62 for deployment of defibrillationcoil electrodes 64, 70 within two chambers of heart 34. Leads 60, 62 mayinclude additional electrodes, such as electrodes 66, 68, 72, 74, forsensing of electrical activity within heart 34. Electrodes 66, 68, 72,74 may form electrode pairs with respective electrodes 52, 56, 58 on IMD48. As in the example of FIG. 1, an accelerometer can be mounted in oneof leads 60, 62 to obtain a heart acceleration signal for use indetecting ischemia. In some embodiments, the heart acceleration signalmay be derived from left-sided leads deployed via the coronary sinus.Also, a pressure sensor may be used in lieu of the accelerometer in someembodiments.

[0048]FIG. 3 is a side view illustrating an implantable lead 76 equippedwith a lead-tip accelerometer. Lead 76 may be configured for use as adiagnostic lead, therapeutic lead, or both, and may be incorporated witha variety of IMDs including those shown in FIGS. 1 and 2. For example,lead 76 may carry sense electrodes, stimulation electrodes, or both. Asshown in FIG. 3, lead 76 may include a distal tip 78, a first section80, and a second section 82. First and section sections 80, 82 includeouter walls 81, 83, respectively, formed of nonconductive, biocompatiblematerial.

[0049] One or more sense or stimulation electrodes may be formed alongthe longitudinal extent of outer walls 81, 83. Distal tip 78 may includean electrode 84, as well as a number of stabilizing tines (not shown inFIG. 3) for securing distal tip member 78 in cardiac tissue upondeployment. In addition, lead 76 may include electrical conductors whichmay be coupled to electrode 84 and an accelerometer assembly mountedwithin second section 82.

[0050]FIG. 4 is a cross-sectional side view of lead 76 shown in FIG. 3.FIG. 4 shows first section 80, second section 82, distal tip 78,electrode 84. Stabilizing tines or other anchoring structure may beadded to distal tip 78, if desired. In the example of FIG. 4, lead 76includes an accelerometer assembly 88 mounted within second section 82adjacent distal tip 78. Accelerometer assembly 88 forms a capsule, andincludes an accelerometer that may be fabricated usingmicroelectromechanical systems (MEMS) technology, providing hightolerance and very small size. Advantageously, accelerometer assembly 88may be used in a bipolar lead system, reducing accelerometer assemblysize and increasing reliability.

[0051] Lead 76 also includes conductors in the form of first and secondconductive coiled conductors 90, 92, which are arranged coaxially alongthe length of the lead. Coiled conductors 90, 92 may be coupled todistal electrode 84 and accelerometer assembly 88 to carry electricalcurrent to and from the electrode and accelerometer assembly to aproximal end of the lead, which may be coupled to an IMD. For example,inner coiled conductor 90 may be coupled to interior components ofaccelerometer assembly 88 via a feedthrough assembly 86. Outer coiledconductor 92 may be coupled to the exterior housing of accelerometerassembly 88, which is electrically conductive and may be formed fromtitanium, and to electrode 84. The accelerometer signal may be producedbetween conductors 90, 92 via an internal accelerometer connection andthe exterior housing connecting, respectively. The signal from electrode84 may be produced between conductor 92 and an electrode on the IMDhousing or “can.” Distal tip 78, first section 80, and second section 82are crimped together at crimp points indicated generally by referencenumerals 94, 96, 98. An adhesive material 99 fills the void withinfeedthrough assembly 86.

[0052] The heart acceleration signal varies as a function of thecontractile force of heart 34. The contractile force is transduced byaccelerometer assembly 88 to produce an electrical heart accelerationsignal that represents the contractility of the heart and, moregenerally, the dynamic mechanical activity of the heart. The contractileforce of heart 34 physically deforms the accelerometer in assembly 88 tochange its electrical properties, and modulate the current passingthrough the accelerometer. Again, an indication of heart contractilitycan be obtained alternatively using a pressure transducer.

[0053] Accelerometer assembly 88 can make use of conventionalaccelerometer technology and may take the form of a piezoelectric,piezoresistive, capacitive, inductive, or magnetic sensor that producesa change in an electrical property with changes in accelerometric forcewithin heart 34. The changes in the electrical property, e.g.,resistance, capacitance, inductance, and the like, in turn produceschanges in the electrical signal produced by accelerometer assembly 88.

[0054] In the example of FIG. 4, accelerometer assembly 88 is mounted atthe tip or distal end of lead 76. Accelerometer assembly 88 could bemounted elsewhere within lead 76, however, provided it can be properlypositioned and oriented to detect accelerometric force produced by thecontractile activity of heart 34. In some embodiments, accelerometerassembly 88 may be formed to have either one, two, or three detectionaxes. In other words, accelerometer assembly 88 may be configured todetect accelerometric force extending in multiple directions as a resultof the contractile force generated by different walls within heart 34.

[0055] In this case, accelerometer assembly 88 may be equipped with amulti-axis accelerometer or multiple accelerometers orientedorthogonally in relation to the respective axes, as well as multipleconductors for obtaining the heart acceleration signal as output fromeach respective accelerometer. As one example, accelerometer assembly 88could include a single conductor line that carries current to multipleaccelerometers, and two or more additional conductor lines that returncurrent from each of the accelerometers to provide separate heartacceleration signal outputs for the different axes. Alternatively, eachaccelerometer may be coupled to the same conductor lines, and producesignals that are time-multiplexed to distinguish the output of eachaccelerometer.

[0056] Detection of heart acceleration along multiple axes may be usefulin determining the location of ischemic tissue. If the heartacceleration signal along one axis is “normal,” i.e., not indicative ofischemia, whereas the heart acceleration signal along another axisindicates a possible episode of ischemia, the location of the ischemictissue can be determined according to the orientation of the axis alongwhich the pertinent accelerometer is aligned.

[0057] In this manner, the ischemic condition can be treated, byintervention of a physician or in an automated manner, and targeted toan appropriate region of heart 34. For example, based on the location ofthe ischemic tissue, electrical stimulation can be delivered to aselected stimulation electrode best suited for treatment of the affectedlocation.

[0058] In addition, the amplitude, frequency, or pulse width ofstimulating current can be controlled according to the affected locationto achieve an optimum therapeutic effect. As a further alternative,determination of the location of ischemic tissue can be used to chooseother types of therapy such as drug delivery, as well as types, dosagesand durations of drug delivery. Also, the location information can becompared to location information recorded in the past to determinewhether the ischemia is occurring in a new location or a location ofprior ischemic episodes.

[0059]FIG. 5 is a block diagram illustrating a system 100 for detectionof ischemia. As shown in FIG. 5, system 100 may include a lead selectorcircuit 102 that selects one or more lead pairs 104, a signal processorcircuit 106, an accelerometer 108, a processor 110, memory 112, atherapy control circuit 112, a therapy delivery system 114, and atelemetry device 116 with an antenna 118. Lead selector circuit 102 maybe controlled by processor 110, and select lead pairs for acquisition ofelectrical signals oriented along multiple detection axes relative toheart 34.

[0060] Processor 110 may take the form of a microprocessor,microcontroller, digital signal processor (DSP) or other programmablelogic device. The electrical signals obtained via the lead pairs can beused to formulate an ECG or EGM for analysis of the PQRST complex and,in particular, the ST segment. Changes in the ST segment can be anindicator of ischemia. Analysis of the dynamic mechanical activity ofthe heart in combination with changes in the ST segment, according tothe invention, can provide a more reliable indication of ischemia.

[0061] Signal processor circuit 106 receives the output of lead selectorcircuit 102 and a heart acceleration signal from an accelerometer 108,which may be deployed in a lead tip as described with reference to FIGS.3 and 4. In other embodiments, signal processor circuit 106 may receivea pressure signal from a pressure transducer. The output of leadselector circuit 102 may be three electrode pair signals, such as RVcoil-can, RV ring-can, and SVC coil-can. In some embodiments, asdiscussed above, accelerometer 108 may produce multiple heartacceleration signals oriented along similar detection axes. Signalprocessor circuit 106 may include a number of sense amplifiers thatamplify the ECG or EGM signals, as well as the heart accelerationsignal.

[0062] In addition, signal processor circuit 106 may include samplingand comparator circuitry for analysis of the electrical signals andheart acceleration signals relative to criteria such as average,peak-to-peak, or total amplitude thresholds. Alternatively, processor110 may digitally sample the signals amplified by signal processorcircuit 106 and perform a software-based analysis of the digitalsignals. Thus, signal processor circuit 106 may include ananalog-to-digital converter that converts the analog signals produced bylead selector circuit 102 and accelerometer 108 into digital samples foranalysis by processor 110. Processor 110 may provide the necessarycontrol and clock signals for operation of signal processor circuit 106.

[0063] A memory 112 is provided for storage of digital samples producedby signal processor circuit 106 and intermediate data stored andretrieved by processor 110. For example, signal processor circuit 106may include a number of buffers that hold digital samples for storage inmemory. Although not illustrated in FIG. 5 for simplicity, processor110, memory 112, and signal processor 106 may communicate via a commondata and instruction bus, as is well known in the art. The digitalsamples may be parameterized, in signal processor circuit 106 orprocessor 110, to produce values for comparison to a predeterminedthreshold. Again, the comparison may take place within discretecircuitry provided by signal processor circuit 106 or via code executedby processor 110. The code may include instructions carried by acomputer-readable medium accessible by processor 110, such as memory 112or other fixed or removable media devices associated with an externalprogrammer/output device communicatively coupled to the processor viatelemetry device 116.

[0064] ECG, EGM, SEA or other electrical signals produced by leadselector circuit 102 can be processed and parameterized to represent avariety of different values useful in the comparison. In one embodiment,the electrical signals may be processed to produce an amplitude value,such as an average, peak-to-peak, or total amplitude, for the ST segmentof the PQRST complex. The ST segment is typically close in amplitude tothe baseline of the ECG or EGM signal sensed between consecutive PQRSTsequences. During episodes of myocardial ischemia, however, the STsegment amplitude may increase or decrease substantially. Thus, bycomparing the amplitude of the ST segment to an amplitude threshold,processor 110 can identify a potential episode of ischemia.

[0065] In addition, processor 110 may be configured to detect a locationof the ischemic condition based on which one of the lead pairs producesan ST segment excursion above the amplitude threshold. In someembodiments, the location may be correlated with one of severalacceleration signals obtained from accelerometer 108 for differentsensing axes.

[0066] An average amplitude may be obtained and represented in a numberof ways such as by computing the average of a series of samples over theperiod of time coincident with the ST segment. A peak-to-peak amplitudefor each signal can be obtained by detection of maxima and minima of theST segment and detection of maxima and minima of a heart accelerationsignal over a duration of time that generally coincides with the STsegment. A total amplitude for each signal can be obtained byintegrating the ST segment and integrating the acceleration signal overa duration of time that generally coincides with the ST segment. Also,because the change in the ST segment may be elevated or depressed duringan ischemic episode, the ST segment parameter may rely on the absolutevalue of the change in the ST segment.

[0067] Because the use of the ST segment as an indicator of ischemia canbe unreliable, processor 110 (and/or signal processor circuit 106) isalso configured to analyze the heart acceleration signal produced byaccelerometer 108. In particular, processor 110 compares a parameterizedvalue representative of the heart acceleration signal, such as anaverage amplitude or integrated amplitude, at a time substantiallycoincident with the ST segment to a pertinent threshold. In this manner,system 100 is capable of correlating the ST segment and the heartacceleration signal for more reliable detection of ischemia.

[0068] By verifying whether the heart acceleration signal (oralternatively a pressure signal) also indicates ischemia, processor 110is able to disregard deviations in the ST segments due to conditionsother than ischemia, e.g., due to changes in the overall PQRST complexcaused by axis shifts, electrical noise, cardiac pacing stimuli, drugs,and high sinus or tachycardia rates that distort the PQRST complex.Consequently, system 100 is capable of reducing the number of falseindications of ischemia, and increasing the reliability of the STsegment as an indicator of myocardial ischemia.

[0069] Based on deviation of the ST segment and the heart accelerationsignal relative to the pertinent thresholds, processor 110 also mayquantify the severity of the ischemic condition. If the ST segment andthe heart acceleration signal both satisfy the pertinent thresholds,processor 110 indicates an ischemic event, and may be programmed toeffect therapeutic action. For example, processor 110 may generate atherapy control signal that causes a therapy control circuit 112 torequest delivery of therapy from a therapy delivery system 114. Therapydelivery system 114 may take, for example, the form of a drug deliverysystem or electrical stimulation system such as a cardioversion ordefibrillation circuit.

[0070] Processor 110 also may indicate to therapy control circuit 112the location of the ischemic tissue and the severity of the ischemiccondition based on the accelerometer signal. Accordingly, therapycontrol circuit 112 may be configured to control therapy delivery system114 based on the indications provided by processor 110. For example,therapy control circuit 112 may select the type of therapy, e.g., drugdelivery and/or electrical stimulation, the dosage, amplitude, andduration of the therapy, as well as the location for delivery of thetherapy, based on the indications of location and severity provided byprocessor 110.

[0071] Processor 110 also may control a telemetry device 116 tocommunicate an indication of the ischemic condition to an externaldevice via antenna 118. Thus, the indication may be a wireless, radiofrequency message that indicates an ischemic condition and, in someembodiments, the location of the ischemic tissue and the severity of theischemic condition. In addition, the IMD itself may have an audiblealarm that notifies the patient when an ischemic episode is occurring.

[0072] The external device, which may be a programmer/output device,advises a physician or other attendant of the ischemic condition, e.g.,via a display or a visible or audible alarm. Also, the ischemic eventsmay be stored in memory in the external device, or within the IMD, forreview by a physician. The components of system 100, with the exceptionof accelerometer 108 and leads 104, may be housed in a common housingsuch as those shown in FIGS. 1 and 2. Alternatively, portions of system100 may be housed separately. For example, therapy delivery system 114could be provided in a separate housing, particularly where the therapydelivery system includes drug delivery capabilities. In this case,therapy control circuit 112 may interact with therapy delivery system114 via an electrical cable or wireless link.

[0073]FIGS. 6A, 6B, and 6C are graphs illustrating an examplerelationship between electrical activity and heart acceleration within acanine heart. In particular, FIG. 6A shows an ECG signal, including theR-wave peak, ST segment and T-wave over a period of time. FIG. 6B showsthe output of a pressure sensor positioned within the left ventricle,e.g., in a lead deployed within the ventricle, over the same period oftime. FIG. 6C shows the output of an accelerometer positioned within theright ventricle, e.g., at the tip of a lead deployed within theventricle, also over the same period of time.

[0074] The heart acceleration signal is characterized by a section 120that generally coincides in time with the ST segment of the ECG signal.Similarly, the pressure signal has a section 121 that coincides with theST segment. In this example, sections 120, 121 are characterized by amomentary positive excursion followed by a negative excursion, whichcorrespond to the contractile forces of the left ventricle. The increasein pressure is due to the pressure developed during contraction. Thepressure drops during relaxation. For the acceleration signal, theincrease is due to the heart's acceleration or vibration duringcontraction, with the acceleration signal occurring during the same timeas the maximum slope of the pressure signal (DP/DT). A secondacceleration signal, typically of a lower amplitude than the firstacceleration signal and corresponding to the maximum negative DP/DT,also can be seen. The waveforms may vary significantly, however,depending on the location of the lead, the accelerometer sensitivityaxis, and other factors.

[0075]FIGS. 6A and 6C also illustrate example amplitude thresholds T1and T2. The thresholds may be used in analysis of the ST segmentamplitude and heart acceleration signal amplitude, respectively. Asimilar threshold can be used for the pressure signal. The thresholdsmay reflect an average amplitude over the duration of the ST segment ora peak-to-peak amplitude. As an alternative, total amplitudes obtained,e.g., by integration of the heart acceleration signal, could be used forcomparison to total amplitude thresholds. In the example of FIGS. 6A-6C,thresholds T1 and T2 represent peak-to-peak amplitude thresholds forcomparison to the maxima and minima of the ST segment and heartacceleration signal, respectively.

[0076]FIGS. 7A, 7B, and 7C are graphs illustrating an examplerelationship between electrical activity and heart acceleration within acanine heart during an episode of ischemia. As shown in FIG. 7A, the STsegment of an ECG or EGM signal may show a significant increase when theheart tissue becomes ischemic. In comparison to FIG. 6A, for example,the amplitude of the ST segment in FIG. 7A is markedly increased, andexceeds the threshold T1, which may be specified by a physician foridentification of ischemic conditions. In FIG. 7B, the amplitude of thepressure signal is decreased relative to that shown in FIG. 6B.

[0077] As shown in FIG. 7C, the heart acceleration signal also shows theeffects of ischemia. Specifically, in a case of ischemia, the section120 of the heart acceleration signal that coincides with the ST segmentis markedly decreased in amplitude relative to FIG. 6C. In this example,section 120 has a peak-to-peak amplitude that is less than the thresholdT2. Thus, when the amplitude of the ST segment exceeds threshold T1 andthe amplitude of the heart acceleration signal drops below threshold T2,an episode of ischemia can be more reliably indicated in accordance withthe invention.

[0078] Again, the amplitudes of the ST segment and heart accelerationsignal, as well as the thresholds T1 and T2, may be peak-to-peak,average, or total amplitudes, or any other parameter deemed reliable indetection of ischemia. The basic technique simply involves analysis ofboth the ST segment and the heart acceleration signal in a correlativemanner to reduce the possibility that changes in the ST segment are dueto factors other than ischemia. This enables a reduction in the numberof false indications.

[0079]FIG. 8 is another graph illustrating changes in heart accelerationin the presence of ischemia. In particular, FIG. 8 illustrates changesin the heart acceleration signal 122 during an experiment in whichischemia is induced in a canine heart. The left axis of the graph showsthe accelerometer peak-to-peak signal, measured in gravitational g's.The bottom axis shows the progression of time. The right axis shows anischemia parameter 124. The ischemia parameter 124 can be derived from,for example, an electrical signal such as the ST segment of an ECG orEGM signal. In particular, ischemia parameter 124 may represent the STsegment change as a percentage of the R-wave amplitude.

[0080] As shown in FIG. 8, following a dobutamine infusion 123, theheart acceleration signal 122 peaks sharply, as the dobutamine induces aforceful contraction in the heart. Later, the heart is subjected toballoon occlusion to intentionally limit the flow of blood, and therebyinduce ischemia. At that time, the ischemia parameter peaks sharply, asindicated by reference numeral 125, whereas the heart accelerationsignal 122 drops noticeably, as indicated by reference numeral 128. Whenthe balloon occlusion is again applied, as indicated by referencenumeral 126, the heart acceleration signal 122 again drops while theischemia parameter peaks. The vertical dashed lines in FIG. 8 denote theduration of the dobutamine infusion, first balloon occlusion, and secondballoon occlusion.

[0081]FIG. 9 is a flow diagram illustrating a process for ischemiadetection. In general, the process may include obtaining an electricalsignal such as an ECG or EGM signal (130) and applying a first criterionto the signal (132). For example, the first criterion may be anamplitude threshold that is compared to an amplitude parameter of theelectrical signal, such as an average, peak-to-peak or total amplitudeof the ST segment of the electrical signal. If the first criterion isnot satisfied, the process returns to evaluation of the electricalsignal (130).

[0082] If the first criterion is satisfied (134), the technique involvesobtaining an accelerometer signal, i.e., a heart acceleration signal(136), and applying a second criterion to the accelerometer signal(138). The second criterion, like the first criterion, may be anamplitude threshold that is compared to an amplitude parameter of theheart acceleration signal, such as an average, peak-to-peak, or totalamplitude in a region that temporally coincides with the ST segment ofthe electrical signal.

[0083] If the second criterion is not satisfied, the process returns toevaluation of the electrical signal (130). If the second criterion issatisfied (140), however, the process indicates an ischemic episode(142). In some embodiments, the process may respond to an indication ofischemia by delivering therapy to the patient (144). For example, theprocess may involve drug delivery or electrical stimulation. The drugdelivery and electrical stimulation may be delivered by an implantablemedical device, including one that is integrated with ischemia detectioncircuitry. Alternatively, drug delivery and electrical stimulation maybe administered to the patient externally.

[0084]FIG. 10 is a flow diagram illustrating another process forischemia detection. The process of FIG. 10 is similar to that of FIG. 9,but illustrates the acquisition of multiple electrical signals fordifferent axes to facilitate determination of the location of ischemictissue. In particular, the process may involve obtaining multiple ECGsignals (144), applying a first criterion to the signals (146), anddetermining whether the criterion is satisfied for any of the signals(148). If so, the process identifies the electrical signals that satisfythe criterion (150), and then obtains the accelerometer signal (152).

[0085] Upon application of a second criterion to the accelerometersignal (154), and satisfaction of that criterion (156), the processindicates an episode of ischemia along with an indication of thelocation of ischemic tissue based on which of the electrical signalssatisfied the first criterion (158), i.e., which of the electricalsignals showed a change in the ST segment indicative of ischemia. Onthis basis, the process may further involve delivery of therapy (160)and, in some embodiments, delivery of therapy to a particular locationwithin the heart, or in a form selected for a particular location.

[0086] Determination of the location of ischemic tissue within the heartalso can be aided by obtaining multiple heart acceleration signals alongmultiple axes. Like the electrical signals, the heart accelerationsignals may indicate ischemia along one axis but not necessarily theothers, enabling isolation of more specific region of ischemia withinthe heart. As with the electrical signals, this may aid in selection ofthe type, level, and focus of the therapy delivered to the patient.

[0087]FIG. 11 is a flow diagram illustrating a process for ischemiadetection in greater detail. As shown in FIG. 11, the process mayinvolve analysis of an electrical signal such as an ECG or EGM signal toidentify the ST segment (162). The ST segment may be parameterized(164), e.g., as a peak-to-peak amplitude, average amplitude, or totalamplitude, and compared to an amplitude threshold T1 (166). If the STsegment amplitude exceeds the threshold T1, there is a potentialischemic condition.

[0088] To more reliably confirm the ischemia, the process involvesobtaining an accelerometer signal (168), parameterizing theaccelerometer signal (170), and comparing it to an amplitude thresholdT2 (172). If the accelerometer signal amplitude drops below thethreshold T2 (172), a contractility change is confirmed in addition tothe increase in the ST segment, providing a more reliable indication ofischemia. On this basis, the process indicates an ischemic condition(174) and may use the indication as the basis for delivery of therapy(176) to the patient.

[0089] Amplitude thresholds are described herein for purposes ofexample, and are not to be read as limiting of the invention as broadlyclaimed. Other signal parameters may be appropriate for evaluation inidentifying ischemia. Also, it is noted that exceeding a given thresholdmay refer to a change that results in an increase above or below acertain level, for example, as described with reference to the graphs ofFIGS. 6, 7, and 8. Specifically, in some cases, ischemia may beindicated by an increase in the ST segment amplitude and a decrease inthe heart acceleration signal at the time of the ST segment. Also, insome embodiments, the electrical and acceleration signals could becombined into a single parameterized value that is compared to a singlethreshold value to determine whether an ischemic episode is indicated.

[0090] The use of a signal indicative of dynamic mechanical heartactivity to confirm an episode of ischemia indicated by the ST segmentof an electrical signal can provide a number of advantages includingmore reliable indication of ischemia, avoidance of false indications andunnecessary administration of treatment. In addition, the heartacceleration signal may be useful, alone or in combination with theelectrical signal, in more reliably quantifying the contractile functionof the heart, and hence the degree of ischemia, providing a standard forthe type or amount of therapy delivered to the patient.

[0091] In addition, a multi-dimensional heart acceleration signal, aloneor in combination with the electrical signal, can be used to betteridentify the location of ischemic tissue. In effect, the use of amulti-axial accelerometer in a lead tip can detect axis shift due topostural changes and add sensitivity to the ischemia detection. Themulti-axial accelerometer signals can be combined in a logical ORfashion to increase sensitivity to ischemia, or combined in a logicalAND fashion to increase specificity, i.e., in terms of the location ofthe ischemic tissue. In addition, relative changes in the orthogonalaccelerometer signals can be used to more narrowly identify the locationof ischemic tissue.

[0092] Various embodiments of the invention have been described.Alternative embodiments are conceivable. Rather than an accelerometer,for example, other sensors such as the pressure transducer describedherein may be employed to obtain a signal indicative of cardiaccontractility. Additionally, the maximum value of the first derivativeof the pressure signal, often called the maximum DP/DT, can be used toassess the cardiac contractility and be used a signal indicative ofischemia. In particular, a blood pressure or velocity transducer mayprovide a signal useful in deriving a measure of cardiac contractility.These and other embodiments are within the scope of the followingclaims.

1. A method for detecting myocardial ischemia, the method comprising:obtaining a first signal indicative of dynamic mechanical activity of aheart; obtaining a second signal indicative of electrical activity ofthe heart; and detecting myocardial ischemia based on both the firstsignal and the second signal.
 2. The method of claim 1, furthercomprising detecting myocardial ischemia when the first signal and thesecond signal both satisfy criteria for indication of ischemia.
 3. Themethod of claim 2, wherein the criteria include a change in the firstsignal having a first predetermined relationship to a first thresholdand a change in the second signal having second predeterminedrelationship to a second threshold.
 4. The method of claim 3, whereinthe first predetermined relationship is a decrease in the first signalthat drops below the first threshold, and the second predeterminedrelationship is an increase in the second signal that exceeds the secondthreshold.
 5. The method of claim 1, further comprising, when myocardialischemia is detected, generating a signal for delivery of therapy toalleviate effects of the ischemia within the heart.
 6. The method ofclaim 1, further comprising, when myocardial ischemia is detected,storing information about the myocardial ischemia for review by aphysician.
 7. The method of claim 1, further comprising, when myocardialischemia is detected, notifying the patient.
 8. The method of claim 1,further comprising, when myocardial ischemia is detected, deliveringtherapy to alleviate effects of the ischemia within the heart.
 9. Themethod of claim 1, further comprising, when myocardial ischemia isdetected, delivering therapy to a patient including at least one of drugdelivery, electrical stimulation, modification of ongoing electricalstimulation, and a combination of drug delivery and electricalstimulation.
 10. The method of claim 1, wherein obtaining the firstsignal includes obtaining a heart acceleration signal from anaccelerometer deployed within the heart.
 11. The method of claim 1,wherein obtaining the first signal includes obtaining a heartacceleration signal from an accelerometer integrated in a tip of animplanted lead deployed in the heart.
 12. The method of claim 11,wherein the lead is a therapeutic lead coupled to an implanted medicaldevice for delivery of electrical stimulation to the patient.
 13. Themethod of claim 11, wherein the lead is a diagnostic lead coupled to animplanted medical device for acquisition of diagnostic data.
 14. Themethod of claim 1, wherein obtaining the first signal includes obtaininga pressure signal from a pressure transducer deployed within the heart.15. The method of claim 1, further comprising obtaining the secondsignal from a set of internal leads implanted in the heart.
 16. Themethod of claim 1, further comprising obtaining the second signal from asubcutaneous electrode array.
 17. The method of claim 1, furthercomprising obtaining the second signal from a set of external leads incontact with the body surface of a patient.
 18. The method of claim 1,further comprising: comparing a change in a first parameter associatedwith the first signal to a first threshold; comparing a change in asecond parameter associated with the second signal to a secondthreshold; and indicating myocardial ischemia when the change in thefirst parameter exceeds the first threshold and the change in the secondparameter exceeds the second threshold.
 19. The method of claim 18,wherein the first signal parameter represents an amplitude of the firstsignal.
 20. The method of claim 18, wherein the first signal parameterrepresents an integral of the first signal during a period of timeproximate to the R-wave.
 21. The method of claim 18, wherein the secondsignal is one of an electrocardiogram and an electrogram, and the secondsignal parameter is an amplitude of an ST segment of the electricalsignal measured relative to the isoelectric level of the electricalsignal.
 22. The method of claim 21, further comprising indicatingmyocardial ischemia when the first signal parameter varies from a firstlevel by a first amount and the second signal parameter varies from asecond level by a second amount.
 23. The method of claim 1, furthercomprising: generating a parameter based on both the first signal andthe second signal; and detecting myocardial ischemia based on comparisonof the parameter to a threshold.
 24. The method of claim 1, furthercomprising quantifying a degree of ischemia based on the first signaland the second signal.
 25. The method of claim 1, further comprisingdetermining a location of ischemic tissue based on the first signal andthe second signal.
 26. The method of claim 1, wherein the first signalincludes a plurality of first signals, each of the first signalsindicating dynamic mechanical activity of the heart along one of aplurality of axes, the method further comprising determining a locationof ischemic tissue based on the plurality of first signals.
 27. Themethod of claim 26, wherein the second signal includes a plurality ofsecond signals, each of the second signals indicating electricalactivity of the heart along one of a plurality of axes, the methodfurther comprising determining a location of ischemic tissue based onthe plurality of first signals and the plurality of second signals. 28.A system for detecting myocardial ischemia, the system comprising: afirst sensor that generates a first signal indicative of dynamicmechanical activity of a heart; a second sensor that obtains a secondsignal indicative of electrical activity of the heart; and a processorthat detects myocardial ischemia based on both the first signal and thesecond signal.
 29. The system of claim 28, wherein the processor detectsmyocardial ischemia when the first signal and the second signal bothsatisfy criteria for indication of ischemia.
 30. The system of claim 28,wherein the criteria include a change in the first signal having a firstpredetermined relationship to a first threshold and a change in thesecond signal having second predetermined relationship to a secondthreshold.
 31. The system of claim 30, wherein the first predeterminedrelationship is a decrease in the first signal that drops below thefirst threshold, and the second predetermined relationship is anincrease in the second signal that exceeds the second threshold.
 32. Thesystem of claim 28, wherein the processor generates, when myocardialischemia is detected, a signal for delivery of therapy to alleviateeffects of the ischemia within the heart.
 33. The system of claim 28,wherein the processor, when myocardial ischemia is detected, storesinformation about the myocardial ischemia for review by a physician. 34.The system of claim 28, wherein the processor, when myocardial ischemiais detected, notifies the patient.
 35. The system of claim 28, whereinthe processor, when myocardial ischemia is detected, controls deliveryof therapy to alleviate effects of the ischemia within the heart. 36.The system of claim 35, wherein the therapy includes at least one ofdrug delivery, electrical stimulation, and a combination of drugdelivery and electrical stimulation.
 37. The system of claim 28, whereinthe first sensor includes an accelerometer integrated with animplantable lead.
 38. The system of claim 28, wherein the lead is atherapeutic lead coupled to an implantable medical device for deliveryof electrical stimulation to the heart.
 39. The system of claim 38,wherein the lead is a diagnostic lead coupled to an implantable medicaldevice for acquisition of diagnostic data.
 40. The system of claim 28,wherein the first sensor includes a pressure transducer integratedwithin an implantable lead.
 41. The system of claim 28, wherein thesecond sensor includes electrodes arranged for acquisition of an ECG,EGM, or SEA signal.
 42. The system of claim 41, wherein at least some ofthe electrodes are carried by implantable leads.
 43. The system of claim41, wherein the electrodes are configured for external use relative to asurface of a human body.
 44. The system of claim 28, wherein theprocessor is programmed to compare a change in a first parameterassociated with the first signal to a first threshold, compare a changein a second parameter associated with the second signal to a secondthreshold, and indicate a myocardial ischemia when the change in thefirst parameter exceeds the first threshold and the change in the secondparameter exceeds the second threshold.
 45. The system of claim 44,wherein the first parameter represents an amplitude of the first signal.46. The system of claim 44, wherein the first signal parameterrepresents an integral of the first signal during a period of timeproximate to the R-wave.
 47. The system of claim 44, wherein the secondsignal is one of an electrocardiogram and an electrogram, and the secondsignal parameter is an amplitude of an ST segment of the electricalsignal.
 48. The system of claim 47, wherein the processor indicatesmyocardial ischemia when the first signal parameter varies from a firstlevel by a first amount and the second signal parameter varies from asecond level by a second amount.
 49. The system of claim 28, wherein theprocessor generates a parameter based on both the first signal and thesecond signal, and detects myocardial ischemia based on comparison ofthe parameter to a threshold.
 50. The system of claim 28, wherein theprocessor quantifies a degree of ischemia based on the first signal andthe second signal.
 51. The system of claim 28, wherein the processordetermines a location of ischemic tissue based on the first signal andthe second signal.
 52. The system of claim 28, wherein the first signalincludes a plurality of first signals, each of the first signalsindicating dynamic mechanical activity of the heart along one of aplurality of axes, and the processor determines a location of ischemictissue based on the plurality of first signals.
 53. The system of claim52, wherein the second signal includes a plurality of second signals,each of the second signals indicating electrical activity of the heartalong one of a plurality of axes, and the processor determines alocation of ischemic tissue based on the plurality of first signals andthe plurality of second signals.
 54. The system of claim 28, furthercomprising a telemetry device for wireless transmission of a messageupon detection of ischemia.
 55. The system of claim 28, wherein thefirst sensor includes a multi-axis accelerometer and the first signalincludes a plurality of heart acceleration signals, each of the heartacceleration signals indicating contractile acceleration along one of aplurality of axes, the processor determining a location of ischemictissue based on the heart acceleration signals.
 56. A method fordetecting myocardial ischemia, the method comprising: obtaining a firstsignal indicative of contractile activity of a heart; obtaining a secondsignal indicative of electrical activity of the heart; and detectingmyocardial ischemia based on both the first signal and the secondsignal.
 57. The method of claim 56, further comprising, when myocardialischemia is indicated, delivering therapy to alleviate effects of theischemia within the heart.
 58. The method of claim 56, wherein thetherapy includes at least one of electrical stimulation and drugdelivery.
 59. The method of claim 56, wherein obtaining the first signalincludes obtaining a heart acceleration signal from an implantedaccelerometer.
 60. The method of claim 59, further comprising obtainingthe accelerometer signal from an accelerometer mounted in a tip of alead implanted in the heart.
 61. A system for detecting myocardialischemia, the system comprising: means for generating a first signalindicative of contractile activity of a heart; means for obtaining asecond signal indicative of electrical activity of the heart; and meansfor detecting myocardial ischemia based on both the first signal and thesecond signal.
 62. The system of claim 61, further comprising means forcontrolling, when myocardial ischemia is indicated, delivery of therapyto alleviate effects of the ischemia within the heart.
 63. The system ofclaim 61, wherein the therapy includes at least one of drug delivery,electrical stimulation, and a combination of drug delivery andelectrical stimulation.
 64. The system of claim 61, wherein the meansfor generating the first signal includes an implanted accelerometer andthe first signal is a heart acceleration signal.
 65. The system of claim64, wherein the accelerometer mounted in a tip of a lead implantable inthe heart.
 66. A computer-readable medium containing instructions forcausing a processor to: obtain a first signal indicative of dynamicmechanical activity of a heart; obtain a second signal indicative ofelectrical activity of the heart; and detect myocardial ischemia basedon both the first signal and the second signal.
 67. A computer-readablemedium containing instructions for causing a processor to: obtain afirst signal indicative of contractile activity of a heart; obtain asecond signal indicative of electrical activity of the heart; and detectmyocardial ischemia based on both the first signal and the secondsignal.